1. Field of the Invention
The present invention relates to the field of position measurement. In particular, the present invention relates to a position-measurement system used to establish and control with high precision the height of a movable platform. More particularly, the present invention relates to such a system based on relatively inexpensive sensors and microprocessors to achieve high precision at relatively low cost. More particularly yet, the present invention relates to such a system that contains as an integral component means to limit the range of travel of the movable platform. Most particularly, the present invention relates to such a system used in conjunction with fork-lifts, elevators, scissors-lifts and the like to provide an inexpensive means to precisely set and control the height of payloads supported by these lift devices.
2. Description of Prior Art
Various types of electronically, pneumatically, and/or hydraulically controlled lifts are widely used in industry to position personnel, materiel, and equipment ["payloads"] at different heights and to move them between desired heights. A simple example is an elevator used to convey payloads between different floors in a factory. More generally, these lifts are used to facilitate payload transfer and assembly (e.g., by lifting a machine being assembled to a convenient height that is well-defined). A particular lift may be portable from job to job or it may be built into a specific industrial operation; the payload capacity of such lifts can range from a few pounds to many tons. For purposes of discussion herein, the payload will be referred to as resting on a "platform," and the height of interest will be referred to as either that of the payload or of the platform or "lift-platform." Typically, platform height is controlled by "UP" and "DOWN" buttons depressed by the lift operator, with the mechanism that actually moves the platform being hydraulically, electrically, or pneumatically based. For economy of text, this discussion will be restricted to hydraulically based lift systems; however, it will be seen that the method and apparatus of the present invention can be applied regardless of the particular nature of the machinery that lifts and lowers the platform. For hydraulically based mechanisms, depressing the UP button typically energizes a pump so as to force pressurized hydraulic fluid into a piston chamber (cylinder) so as to force the piston to move; the moving piston, through linkage coupling it to the platform, raises the platform. When the platform is at the desired height, the UP button is released, which has the effect of halting the fluid flow into the cylinder and preventing that fluid that is already there from leaving the cylinder. Conversely, in this arrangement, depressing the DOWN button, again through a solenoid valve or its equivalent, allows the hydraulic fluid in the chamber to drain out and return to a supply reservoir, while ensuring that no more fluid flows in. Many variations on this are possible. If the operator has allowed the platform to overshoot the desired position, he or she will have to "jog" the UP or DOWN button so as to cause the platform height to zero in on its desired value.
If the lift is being used for manual loading/unloading, or other procedures not requiring precise, repetitive platform positioning, the procedure just described can be perfectly adequate, as long as the target heights, such as those of shelves, are clearly visible to the operator. However, in modern applications, this is often not the case, and the target height can be far above the operator's eye level, making "eyeballing" difficult or impossible. Also, there are many applications where a lift is used to repetitively convey items from one level to another, as from one automatically-operated work station to another. This type of task requires that the payload be delivered to precisely the desired level the first time, with no subsequent jogging up or down, and that it do this many times in the course of a day, or an hour. The prior art relating to lift-positioning that is precise and reproducible tends to be limited to expensive control systems used in conjunction with expensive and/or fixed lift systems. Alternatively, they are limited to applications where one is interested in placing a platform at only a few, discrete heights, such as is the case with elevators used to convey people and equipment between floors in a factory or other building, or between shelves of an inventory-storage structure in a warehouse.
The system of Allen et al. (U.S. Pat. No. 4,122,957) is directed to fork-lifts, to assist the forklift operator to position the payload at well-defined shelf heights that may be out of the range within which he or she can "eye-ball" them. It uses reflecting tape bearing shelf-specific coding on the respective shelves and a photosensor/microprocessor combination tied into the mechanism controlling the motion of the lift. Using this system, the operator can "key in" any desired shelf and the forks will automatically move to the correct level, there to retrieve or deposit some item of interest.
The system of Allen et al., useful though it is for its intended purpose, does not provide accurate height measurement/positioning anywhere but at the discrete shelf heights. Often one wishes to have absolute position determination on a continuous basis, even when the purpose of the lift mechanism, such as an elevator, is to stop at discrete floor levels. Simpson (U.S. Pat. No. 3,483,950) discloses an elevator control that uses a coded tape that moves with the elevator car, passing over a pulley in the machine room where, by photodetector techniques, the control mechanism determines the position of the tape and hence of the elevator car. This is an example of the use of a surrogate-based parameter to monitor the quantity of true interest, namely the height of the platform. The tape contains a unique code for each floor and is capable of carrying coded information regarding intermediate positions for the car. An improvement on the system of Simpson is disclosed in Payne et al. (U.S. Pat. No. 4,427,095), which teaches an elevator-control system dependent on a fixed encoded tape placed along the elevator shaft, the tape code to be detected and read by a photosensor on the elevator car. The system of Payne et al., and its refinement described in Watt et al. (U.S. Pat. No. 5,135,081), do provide a precise and reproducible method for positioning an elevator car at a myriad of positions along the elevator shaft. However, they are by their nature, limited to fixed installations and pretty much to elevators and the like. Furthermore, the resolution of position still depends on how closely the individual encoded-for positions can be imprinted on the tape or other surrogate.
Yuki et al. (U.S. Pat. No. 4,499,541) and several patents cited therein teach the measurement of the angular orientation of a rotary element attached to a forklift-driving shaft as a surrogate for the height of the forks. As must generally be true of all such surrogates, this angular orientation bears a one-to-one relationship with the height of the forks. The angular orientation of this rotary element is thus a "surrogate parameter" for the height of the platform; measuring the former gives you the latter. Thus, one of the embodiments of the method of Yuki involves putting markers around the perimeter of the referenced rotary element, and then deploying a sensor that "looks at" that perimeter and is capable of "seeing" one marker at a time. Either by counting similar markers, or, if each marker is made to be unique, recognizing markers, the sensor is able to detect and transmit information as to the instantaneous orientation of this surrogate element. This information is then used by the rest of the control mechanism to position the platform as desired. The resolution of this type of device is limited by the proximity with which the markers can be placed on the rotary element or, alternatively, the minimum size of sensor employable to read the markers. As is commonly the case with surrogates that are conveniently located and sized, a small error in determining the surrogate parameter will lead to a large uncertainty in the position of the forks.
Another surrogate-parameter-based approach to platform-height measurement and control is disclosed by Ekman (U.S. Pat. No. 4,252,213), which is directed to sky-lifts and the like incorporating two or more hydraulic rams. Indeed, its major thrust is to simplify and otherwise consolidate the means by which the platforms (buckets) of articulated lifts are varied in height and position. The method is based on using one of a variety of parameters, such as the array of hydraulic ram lengths, the array of angles between the articulated arms, and the like. As Ekman points out, given the fixed lengths of the various arms of the lift, the height and position of the platform are in one-to-one correspondence with these alternative sets. By this is meant that if one knows, for example, the angles between all five arms of the articulated lift, then one knows the height of the bucket and also the distance by which the bucket is offset from the center of the vehicle supporting it. Ekman states without further detail that the actual, absolute values of inter-arm angles, ram extensions, etc., whatever the surrogate parameter may be, are directly measured (by, for example, "electric signals"). The implication is that there is no limit to the precision with which the parameter is thereby determined. That being the case, there should be no limit to the resolution with which the bucket position is determinable. This also implies that the measurement of the surrogate-parameter arrays is carried out by relatively expensive means. Finally, it appears from Ekman that the rather complex relationship between the height/offset of the bucket on the one hand and the value of the multiplex parameter on the other (the "geometric model") is derived for each different lift and stored in the microprocessor that constitutes the heart of the control unit. In other words, there seems to be no suggestion that the microprocessor itself is used to establish the correlation.
Ochoa et al. (U.S. Pat. No. 5,695,173) discloses a scissors-lift incorporating an electronic platform-height control. The heart of this system is a series of optical sensors mounted on the platform where they are laterally deployed. Corresponding to each of the optic sensors is a series of vertically deployed light-emitters (actually reflectors) mounted on a fixed plate facing the sensor-displaying portion of the platform. By selecting a particular sensor to control the motion/position of the platform, one can with the system of Ochoa et al. cause the platform to move between a series of predetermined heights. By its nature, the system of Ochoa et al. cannot provide a general position measurement system, nor does it purport to. Rather, it provides merely a means for a closed-loop height adjustment around a small number of heights, at each of which is located a target that is directly connected to the mechanical height-setting control. In that sense, the resolution of the Ochoa et al. device is equal to the spacing of the optical sensors. The Ochoa et al. device has a further disadvantage in that the sensors are mounted directly on the platform. This placement of the sensors can impede full usage of the platform and increases the possibility of the sensors being damaged in course of operating the platform.
Separate from the need to accurately and reproducibly position the lift platform at a desired height is the requirement to keep the platform from exceeding its normal range, so as to prevent damage to the underlying mechanism. Traditionally, this requirement has been met by installing simple limiting switches at the physical locations of the upper and lower extremes of the allowable range. When the platform bumps into either of the switches, power is interrupted from the UP or DOWN control, respectively. Unfortunately, as with all exposed electromechanical devices, these limit switches are subject to damage and resulting malfunction, which can lead to costly damage to the lift mechanism. The prior art relating to precision position-control, including the system of Ekman, appears to still rely on some type of crude limiting means independent of the precision position-control.
Related to the function of the limiting switches in the traditional lifts are the "home switches" that many lift applications require, switches that signal a return of the lift to some particular "home" position, for example, to receive the next delivery of equipment or inventory. Any type of automatic platform-height control would ideally incorporate a limit-of-travel means and the option of causing the platform to return to its "home" position as a default.
Therefore, what is needed is a rugged yet inexpensive system for height-measurement and control of lift platforms, including those of non-fixed lifts such as fork-lifts and scissors-lifts. What is further needed is such a system that provides such measurement and control throughout the entire normal height range of a lift-platform, rather than just at discrete heights. What is yet further needed is such a system that incorporates end-of-travel limit switches and home switches and, in general, automatic-movement activators triggerable at any pre-determined key platform height.